Transposing a matrix - c

Question

I want to transpose a matrix, its a very easy task but its not working with me :
UPDATE
I am transposing the first matrix and
storing it in a second one
The two
arrays point to the same structure
I
need two arrays (target and source)
so I can display them later for
comparison.
struct testing{
int colmat1;
int rowmat1;
float mat[64][64];
};
int testtranspose(testing *test,testing *test2){
int i,j;
test2->colmat1 = test->rowmat1;
test2->rowmat1 = test->colmat1
for(i=0;i<test->rowmat1;i++){
for(j=0;j<test->colmat1;j++){
test2->mat[i][j] = test->mat[i][j];
}
printf("\n");
}
}
I thought this is the correct method of doing it, but apparently for a matrix such as :
1 2
3 4
5 6
7 8
I get :
1 2 0 0
3 4 0 0
What is the problem ?
Please help,
Thanks !

Answer 1

To transpose the matrix, you need to change rows and columns. So you need to use:
targetMatrix[i][j] = sourceMatrix[j][i];
Note how the order of i,j is changed, since one matrix's rows are another's columns.
By the way, instead of (*a).b, you can write a->b. This is the normal way of accessing a field of a struct pointer.

Answer 2

Try this...
struct testing{
int colmat;
int rowmat;
float mat[64][64];
};
int testtranspose(testing *test,testing *test2){
int i,j;
test2->colmat = test->rowmat;
test2->rowmat = test->colmat;
for(i=0;i<test->rowmat;i++){
for(j=0;j<test->colmat;j++){
test2->mat[j][i] = test->mat[i][j];
}
}
return 0;
}
int printmat(testing* mat)
{
for(int i=0;i<mat->rowmat;i++)
{
printf("\n");
for(int j=0;j<mat->colmat;j++)
printf((" %f"),mat->mat[i][j]);
}
return 0;
}
// 2
// main.cpp
int _tmain(int argc, _TCHAR* argv[])
{
testing mat1, mat2;
memset(&mat1,0,sizeof(testing));
memset(&mat2,0,sizeof(testing));
mat1.colmat =2;
mat1.rowmat =3;
for(int i=0;i<mat1.rowmat;i++)
{
for(int j=0;j<mat1.colmat;j++)
mat1.mat[i][j] = (float)rand();
}
printmat(&mat1);
testtranspose(&mat1,&mat2);
printmat(&mat2);
getchar();
}

Answer 3

I am new to C / C++ (3rd day or so :) ) and I had the same problem. My approach was slightly different in that I thought it would be nice to have a function that would return a transposed matrix. Unfortunately, as I found out, you cannot return a an array nor pass an array to a function in C++ (let alone a double array), but you can pass / return a pointer which works similar to an array. So this is what I did:
int * matrix_transpose(int * A, int A_rows, int A_cols){
int * B;
int B_rows, B_cols;
B_rows = A_cols; B_cols= A_rows;
B = new int [B_rows*B_cols];
for(int i=0;i<B_rows;i++){
for(int j=0;j<B_cols;j++){
B[i*B_cols+j]=A[j*A_cols+i];
}
}
return B;
};
The trick was in dynamic arrays. I used A_rows and B_rows as separate names (you can use only rows and cols) in order to make the problem less intricate and less confusing when reading code.
B = new int [rows*cols] // This is cool in C++.